Metal ones are cheap and easy to get at local gas stations. The one benefit is that they won't blow away easily in the wind. And when dirt hits them, they actually make a nice "ting" sound your horse will need to get used to. (Good training purpose) However, it really hurts when that metal hits your shin.
And most only come as black metal so if you want bright colors you'll need to purchase barrel covers or paint the metal.
Plastic is much easier to handle because it is lighter, and it doesn't hurt near as much when you hit a shin on it. They're usually blue or white, and bright colors can be scary to a horse, so that can help train them there. But you will need to fill them with water or sand so they don't blow away in the wind.
I grew up with metal barrels. Then we acquired some plastic ones. Now I'm on my own and I just went to the local gas station and bought 3 metal barrels for $15 each.